Conventionally in a construction machine, engine lubricating oil replacement is based on an engine operating time as measured by a hour meter. A user must compare the measured engine operating time to a replacement operating time defined by the construction machine manufacturer. In a vehicle or the like, replacement is performed on the basis of a period of time specified by the vehicle manufacturer or a traveling distance. This method, however, has a problem that a lubricating oil which remains useful may be wastefully replaced with new oil depending upon a condition for use or that lubricating oil subject to severe conditions may not be immediately replaced with new oil, which reduces the working life of the engine.
Accordingly, as a method of deciding a time for replacing a lubricating oil of an engine, there is provided Japanese Patent Publication No. 60-44490 of 1985. In this publication, there is disclosed an apparatus for replacing an engine oil comprising a sensor for detecting the number of revolutions of an engine, a sensor for detecting a load, and a calculating section for calculating an amount of tramp soot (hereinafter "soot") within the oil on the basis of output values of these sensors. The Japanese reference further discloses an apparatus for integrating and storing the amount of soot, in which an alarm is produced in order to instruct an operator to replace the oil when the integrated total amount of soot in oil exceeds a predetermined amount.
The working life of lubricating oil commonly exhibits the following characteristics. In one case, increased engine wear is caused by an increase of oil viscosity due to the soot contained in the oil and in addition to a decrease of oil alkali value. In the other case, increased engine wear is caused by a bearing seizing to an increase of the total acid value of the lubricating oil in addition to an increase of sludge caused by an increased acidity in the lubricating oil.
Thus, the invention disclosed in Japanese Patent Publication No. 60-44490 for detecting soot has the following problems:
(1) if an engine is radically loaded, as in a construction machine, the amount of smoke from the engine is rapidly increased, consequently the amount of soot in the engine lubricating oil sharply changes, thereby affecting the ability to accurately predict the amount of soot content within the lubricating oil; PA1 (2) if a load on an engine is greatly changed as in a construction machine, a large capacity calculating section to calculate a soot content within an engine lubricating oil is needed and a control unit cannot commonly follow the variation, thereby again affecting the ability to accurately predict the amount of soot content within the lubricating oil; PA1 (3) the amount of soot changes depending upon an operating condition (e.g., a temperature of the lubricating oil or the like) of the engine, further impeding the ability to accurately predict the content of soot within the lubricating oil; and PA1 (4) it is difficult to grasp the total acid value in the engine lubricating oil, thus also affecting the ability to accurately predict the lubricating oil soot content.